Bedeviling presidents since George Washington, the U.S. Senate has a well-earned reputation as the saucer that cools the House’s hot tea. The foremost example is the filibuster, an institution now under fire in the case of Supreme Court nominee Neil Gorsuch. But the filibuster’s colorful cousin could come under renewed threat in the coming months with widespread ramifications for the nation’s federal courts. Donald Trump’s legacy on the judiciary, in many ways, rests on the fate of the blue slip.

It’s tough to ascertain the first time a senator wrote his preference for a nominee from his state on a piece of blue paper, but historians think the practice dates to at least 1917. The concept behind it can be traced to 1789, when President George Washington nominated Benjamin Fishbourn to be naval officer for the Port of Savannah. Georgia Sen. James Gunn didn’t like Fishbourn, and the rest of the Senate linked arms to block Washington’s choice. The president backed down. This episode established the concept of “senatorial courtesy” on nominees in their states — be they judges, U.S. attorneys or federal marshals. “It’s an important prerogative that each home-state senator, regardless of the president, really clings to, because it gives them control,” says Glenn Sugameli, an environmental attorney in Washington who tracks judicial nominations.

As the Obama administration consulted with Republican senators on nominees, we expect the Trump administration to do the same.

Sen. Dianne Feinstein, D-Calif.

Even though it’s not enshrined in law, it has become standard practice for senators to come up with names to send to the White House before the administration vets them and issues formal nominations. Then the nominees are sent to the Senate Judiciary Committee, where the blue slip practice takes hold: Home-state senators send the committee an opinion on the nominees. Without two thumbs up, the nominee won’t get a committee hearing — at least, that’s how things have gone in recent years.

A 1917 Senate blue slip for U.V. Whipple, a candidate for district judge for the southern district of Georgia, signed by Sen. Thomas Hardwick.

While you will hear paeans to a century-old tradition, often loudest from senators in the minority, the custom’s application varies by committee chairman. Until 1956, according to the nonpartisan Congressional Research Service, a blue slip was merely advisory and had no real power. Since then, most chairmen have deferred to the home-state senators, but had loopholes in their blue slip policies so that a few nominees could sneak through. But when Democrats held the Senate during the Obama years, Judiciary Chairman Patrick Leahy, D-Vt., was a blue slip absolutist — to the frequent consternation of liberal activist groups and even his own majority leader.

Now both the presidency and the Senate are in Republican hands, and rumblings are beginning to be heard from the right for more flexibility. “If Trump is to make his appointments meaningful, he would have to depart from long-standing tradition that gives home-state senators major input on nominees and allows them to potentially scuttle the nomination,” Daniel Horowitz wrote this year on the right-wing website Conservative Review.

But it’s not up to Trump. The decision falls to the current judiciary chairman, Sen. Chuck Grassley, an Iowa Republican with an independent streak and an institutionalist bent. Grassley has praised the custom in the past, and his spokesman told The Wall Street Journal in November that Grassley would honor the blue slip process. (Grassley’s office did not respond to OZY’s requests for comment.) “There may be a rude awakening” for Trump, says Carl Tobias, professor at the University of Richmond’s law school and an expert on the judicial nomination process.

Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, speaks to the press in support of the nomination of Neil Gorsuch to the U.S. Supreme Court on March 29, 2017, in Washington, D.C.

So far, there have been no opportunities for a clash. Aside from Gorsuch — Supreme Court nominees are a whole different ballgame — Trump has nominated just one federal judge, a Kentuckian with two Republican senators. But expect a fight over the Ninth Circuit Court of Appeals. Derided as the “ninth circus” by conservatives for its liberal bent, the nation’s largest appeals court covers an area from Montana to Guam. The Ninth Circuit has already drawn Trump’s ire by rejecting his travel ban.

There are four current openings on the Ninth Circuit, and three of them come from states with two Democratic senators — California, Oregon and Hawaii. Those senators “are pretty much to the left, and so they seem to be cruising for a fight,” Tobias says. Appeals courts carry vast power; their decisions create precedents, and the Supreme Court is taking relatively few cases. The battles over those slots tend to be more partisan, and the nominations more White House driven.

In the case of California’s empty Ninth Circuit slot, Barack Obama’s nominee, Lucy Koh, was held up as part of a Republican blockade that left Trump with double the number of judicial vacancies that Obama inherited. California Sen. Dianne Feinstein, the top Democrat on the Judiciary Committee, pointed to Grassley’s past support of blue slips and said she would come up with candidates for openings in her state via a bipartisan commission — a common practice among senators. “As the Obama administration consulted with Republican senators on nominees, we expect the Trump administration to do the same,” Feinstein told OZY via a spokeswoman.

With 125 openings among 890 authorized federal judgeships, with two nominees pending, conservatives are eager for Trump to tip the federal judiciary to the right. The White House did not respond to requests for comment, but Politico recently reported that the administration’s early focus is on judges in their late 30s and early 40s who will stay planted on the bench for decades. We’ll see what a parochial gang of senators has to say about that.